A multi-focal contact lens formed with a plurality of distance portions for seeing a distant place and a plurality of near portions for seeing a near place arranged alternately in concentric zone shape has been proposed in Japanese Published Unexamined (Kokai) Patent Application No. 59-146200, for instance. When the user uses the contact lens as described above, the user can select one of the distance portions and the near portions by the user' consciousness. In other words, since the user can properly use one of the distance portions and the near portions naturally and smoothly, these contact lenses are useful for the users.
In more detail with reference to FIG. 6, the contact lens 1 as described above is provided with a front curve 2 and a base curve 3 fitted to a curved surface of the user' cornea. The front curve 2 is formed with a plurality of distance portion curved surfaces F.sub.1, F.sub.2, . . . and a plurality of near portion curved surfaces N.sub.1, N.sub.2, . . . . These distance and near portion curved surfaces are arranged alternately and repeatedly in concentric zone shape.
In the conventional contact lens of this sort, a plurality of distance portion curved surfaces F.sub.1, F.sub.2, . . . and a plurality of near portion curved surfaces N.sub.1, N.sub.2, . . . are formed as follows:
Here, a radius of curvature of the distance portion curved surface is represented by r.sub.F, and a radius of curvature of the near portion curved surface is represented by r.sub.N. First, a circle with a radius r.sub.F is described with a point P on an optical axis (z axis) as its center to obtain an intersection point O.sub.F1 between the described circle and the optical axis. The obtained intersection point O.sub.F1 is determined as a center of curvature of the distance portion curved surface F.sub.1. Then, a circle with a radius r.sub.F is described with the curvature center O.sub.F1 as its center to obtain an intersection point P.sub.F1 between the described circle and a straight line l.sub.F1 parallel to the optical axis. This parallel straight line l.sub.F1 is used to determine a predetermined radial zone width of the distance portion curved surface F.sub.1. Further, a circle with a radius r.sub.N is described with a point P.sub.F1 as its center to obtain an intersection point O.sub.N1 between the described circle and the optical axis. The obtained intersection point O.sub.N1 is determined as a center of curvature of the near portion curved surface N.sub.1. Then, a circle with a radius r.sub.N is described with the curvature center O.sub.N1 as its center to obtain an intersection point P.sub.N1 between the described circle and a straight line l.sub.N1 parallel to the optical axis. This parallel straight line l.sub.N1 is used to determine a predetermined radial zone width of the near portion curved surface N.sub.1.
In the same way as above, a circle with a radius r.sub.F is described with a point P.sub.N1 as its center to obtain an intersection point with the optical axis. The obtained intersection point is determined as a center O.sub.F2 of the curvature of the distance portion curved surface F.sub.2. With this obtained curvature center O.sub.F2 as its center, a circle with a radius r.sub.F is described to obtain an intersection point P.sub.F2 between the described circle and a straight line l.sub.F2 parallel to the optical axis and thus to determine a predetermined radial zone width of the distance portion curved surface F.sub.2. Further, a circle with a radius r.sub.N is described with a point P.sub.F1 as its center to obtain an intersection point with the optical axis. The obtained intersection point is determined as a center O.sub.N2 of the curvature of the near portion curved surface N.sub.2.
FIG. 4 shows the centers O.sub.F1, O.sub.F2, . . . of the curvatures of the distance portion curved surfaces F.sub.1, F.sub.2, . . . and the centers O.sub.N1, O.sub.N2, . . . of the curvatures of the near portion curved surfaces N.sub.1, N.sub.2, . . . obtained as described above. FIG. 4 indicates that the centers O.sub.F1, O.sub.F2, . . . of the curvatures of the distance portion curved surfaces F.sub.1, F.sub.2, . . . are distributed in the z-axis direction so as to be shifted in sequence in the direction from the front curve 2 to the base curve 3. On the other hand, the centers O.sub.N1, O.sub.N2, . . . of the curvatures of the near portion curved surfaces N.sub.1, N.sub.2, . . . are distributed in the direction opposite to the z-axis direction so as to be shifted in sequence in the direction from the base curve 3 to the front curve 2.
As a result of this, as shown in FIG. 5, light rays parallel to the optical axis and incident upon the respective distance portion curved surfaces F.sub.1, F.sub.2, . . . are focused at different distance portion focal points F.sub.F1, F.sub.F2, . . . of the respective distance portion curved surfaces F.sub.1, F.sub.2, . . . , respectively, without focusing at a single focal point. In the same way, light rays parallel to the optical axis and incident upon the respective near portion curved surfaces N.sub.1, N.sub.2, . . . are focused at different near portion focal points F.sub.N1, F.sub.N2, . . . of the respective near portion curved surfaces N.sub.1, N.sub.2, . . . , respectively, without focusing at a single focal point. In other words, in the conventional contact lens, there exists spherical aberration, thus raising a problem in that it is impossible to obtain a clear image. Further, in FIG. 5, it should be noted that the arrangement sequence of the distance portion focal points F.sub.F1, F.sub.F2, . . . is opposite to the arrangement sequence of the centers O.sub.F1, O.sub.F2, . . . of the curvatures of the distance portion curved surfaces F.sub.1 , F.sub.2 , . . . , because the spherical aberration at the peripheral portions of the contact lens 1 is large.
In addition, the conventional contact lens involves another problem in that in use of the contact lens, it is impossible to obtain a desired field of vision (as wide as a normal naked eye) and a natural peripheral field of vision.
Further, in the case of the above-mentioned contact lens, since the lens is so designed and so manufactured that the optical energy incident upon the distance portions is equal to that incident upon the near portions, when the lens is used as the near portion lens (e.g., for reading or desk working), the light rays passed through the distance portions are not focused at the retina, and when used as the distance portion lens (e.g., for outdoor use), the light rays passed through the near portions are not focused at the retina. In other words, since the function of the lens formed with both the distance portions and the near portions is reduced half in indoor use or in outdoor use, thus the conventional contact lens being not easy to use.
Furthermore, in the conventional method of manufacturing the contact lens 1 provided with the front curve 2 formed with the distance portion curved surfaces and the near portion curved surfaces arranged alternately and repeatedly in concentric zone shape, since the front curve 2 is formed with the distance and near portion curved surfaces with different radii of curvatures (not a single smoothly curved surface), there exists a problem in that it is impossible to polish the surface of the contact lens uniformly including the boundary portions between the adjoining distance and near portion curved surfaces.
With these problems in mind, therefore, it is the primary object of the present invention to provide a contact lens having a front curve formed with a plurality of distance portion curved surfaces and a plurality of near portion curved surfaces arranged alternately and repeatedly in concentric zone shape without spherical aberration, while solving the problems involved in the conventional contact lens.
Further, another object of the present invention is to provide a contact lens having a desired field of vision, for instance, as wide a field of vision as the normal naked eye and further natural in the peripheral field of vision and additionally clear in eyesight.
Further, another object of the present invention is to provide a contact lens provided with the performance of almost a single focal lens but with the functions of both the distance portion lens and the near portion lens by using the lens properly according to indoor use and outdoor use.
Furthermore, the other object of the present invention is to provide a method manufacturing a contact lens which can be polished uniformly including the boundary portions between the two adjoining distance portion curved surfaces and the near portion curved surfaces.